1. Field of the Invention
The present invention relates to an optical amplifier glass, particularly a broadband optical amplifier glass which is operable in a wavelength range of from 1.55 to 1.65 xcexcm.
2. Discussion of Background
For the purpose of application to the optical communication field, there have been research and development of an optical fiber amplifier using as an optical amplification medium an optical fiber having a rare earth element doped to the core, and an Er (erbium)-doped optical fiber amplifier (EDFA), and their application to an optical communication system is being actively pursued. On the other hand, to cope with diversification of communication services expected in future, a wavelength division multiplexing communication system (WDM) has been proposed to increase the transmission capacity. As the number of wavelength division multiplexing channels increases, the transmission capacity will increase. Application of EDFA to such a wavelength division multiplexing transmission system is also being studied. As EDFA so far proposed, an Er-doped quartz type fiber and an Er-doped fluoride fiber are known.
In the case of a conventional Er-doped quartz type fiber, the wavelength dependency of the gain is sharp, and the wavelength width wherein an adequate gain is obtainable, is as narrow as from about 10 to 30 nm. Consequently, so far as such conventional EDFA is employed, the number of wavelength division multiplexing channels is limited to a level of from 30 to 40 channels.
If EDFA showing a flat gain within a wider wavelength range, is realized, it is expected to be able to broaden the useful signal wavelength and thereby to substantially improve the transmission capacity. Accordingly, development of such EDFA is desired.
In order to solve such problems, an optical amplifier which can be used in a wide wavelength range has been proposed wherein amplifiers having different amplification gain characteristics to wavelengths, are arranged in series or in parallel, but there has been a problem such that the structure tends to be cumbersome, or in the vicinity of the center of the wavelength range, there is a region where no amplification is possible. Further, JP-A-8-110535 proposes a tellurite type glass as a glass capable of amplification in a broadband range. However, such tellurite type glass usually has a low glass transition point and is thermally unstable. In order to improve the amplification gain of an optical amplifier, it is necessary to let a high intensity excited laser beam enter into the glass, but such glass was likely to be thermally damaged by the strong laser beam.
It is an object of the present invention to solve the above-mentioned problems and to provide an optical amplifier glass having a high glass transition point and having a wide wavelength width wherein the gain is obtainable.
The present invention provides an optical amplifier glass comprising a matrix glass containing Bi2O3 and at least one of Al2O3 and Ga2O3, and Er doped to the matrix glass, wherein from 0.01 to 10% by mass percentage of Er is doped to the matrix glass which has a total content of Al2O3 and Ga2O3 of at least 0.1 mol %, a content of Bi2O3 of at least 20 mol %, a refractive index of at least 1.8 at a wavelength of 1.55 xcexcm, a glass transition temperature of at least 360xc2x0 C. and an optical basicity of at most 0.49.
With an optical amplifier glass having Er-doped to a matrix glass, optical amplification is carried out by means of stimulated emission transition from the 4I13/2 level to the 4I15/2 level of Er. The present inventors have found that the wavelength width wherein the optical amplification gain is obtainable, is dependent on the optical basicity which will be described hereinafter, and have thus arrived at present invention. The wavelength width within which the optical amplification gain is obtainable, will hereinafter be referred to as xe2x80x9cthe gain wavelength widthxe2x80x9d.
The gain wavelength width has heretofore been considered to be as follows. Namely, the gain wavelength width is dependent on the refractive index of the matrix glass, i.e. the gain wavelength width increases as the refractive index increases. This has been explained such that the electric field which Er receives in the matrix glass increases as the refractive index increases, and consequently, the energy level of Er is broadened, whereby the emission spectrum becomes broad.
However, the present inventors have found that the gain wavelength width xcex94xcex is not only dependent on the refractive index of the matrix glass but also strongly dependent on the optical basicity xcex9 of the matrix glass, which will be described hereinafter, i.e. xcex94xcex becomes large as xcex9 becomes small.
With respect to a glass composition represented by mol % of oxide components, the optical basicity xcex9 is defined as follows. Namely, with a glass containing Ci mol % of an oxide of component i,
xcex9=1xe2x88x92xcexa3[zixc2x7ri(xcex3ixe2x88x921)/2xcex3i]
xcexa3 indicates summing up with respect to subscript i
xcex3i=1.36(xixe2x88x920.26) 
zi: valency of the cation in the oxide of component i,
ri: the ratio of the number of cations in the oxide of component i to the total number of oxygen in the above xe2x80x9cglass composition represented by mol % of oxide componentsxe2x80x9d,
xi: the Pauling""s electronegativity of an atom bonded to oxygen in the oxide of component i.
For reference, the Pauling""s electronegativities of main atoms are shown below.
Li:1.0, Na:0.9, K:0.8, Mg:1.2, Ca:1.0, Sr:1.0, Zn:1.6, Ba:0.9, B:2.0, Al:1.5, Si:1.8, P:2.1, Ge:1.8, Ga:1.6, Te:2.1, Sn:1.8, Sb:1.9, W:1.7, Pb:1.8, Bi:1.9, and Ti:1.5.
For example, with respect to a glass wherein the oxide of the first component is Bi2O3, and the oxide of the second component is SiO2, and the composition represented by mol % is 20Bi2O3/80SiO2,
z1=3, z2=4
Total number of oxygen=0.2xc3x973+0.8xc3x972=2.2,
r1=0.2xc3x972/2.2, r2=0.8xc3x971/2.2,
x1=1.9, x2=1.8,
xcex9=0.47.
The optical basicity is one which Duffy et al. have proposed as an index of the basicity of glass in J. Am. Chem. Soc., 93 (1971) 6448, and it is one obtainable by a simple calculation from the glass composition without necessity of carrying out the measurement or complex analysis or calculation.
Now, the relation between xcex9 and xcex94xcex will be described based on data.
Glasses A to I having Er doped to a matrix glass having a composition shown by mol % in lines for from Bi2O3 to ZnO in Table 1, were prepared. The amount of Er doped, is shown in the line for Er in Table 1 by mass % based on the matrix glass being 100%. With respect to these glasses, the refractive index n at a wavelength of 1.55 xcexcm, the glass transition point Tg (unit: xc2x0 C.) and the gain wavelength width xcex94xcex (unit: nm) were measured. Further, the optical basicity xcex9 was calculated from the composition. The measuring methods for n, Tg and xcex94xcex were as follows.
n: measured by an Ellipsometer.
Tg: measured by a differential thermal analysis (DTA).
xcex94xcex: excited by a laser beam with a wavelength of 980 nm, and it was obtained from the emission spectrum obtained by this excitation.